Ultrasonic inspection method and apparatus



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Jac ues New T Mme/mu kn FL 29K www United States Patent I 3,303,691ULTRASONIC INSPECTION METHOD AND APPARATUS Louis Beaujard, JacquesMondot, and Marian Kapluszak, Saint Germain-en-Laye, France, assignorsto Institut de Recherches de la Sidrurgie Francaise, SaintGermainen-Laye, France, a professional institution of France Filed Feb.19, 1963, Ser. No. 259,659 Claims priority, application France, Feb. 23,1962, 888,981, Patent 1,323,002 10.Claims. (Cl.73-71.5)

The present invention relates to improvements in ultrasonic inspectionsystems, and more particularly to a novel method of ultrasonicallydetecting flaws in test pieces at an elevated temperature above thepoint of depolarization or the Curie point of the usual piezo-electrictransducer used in such systems.

In most such coventional systems, a refractory material is interposedbetween the piezo-electric crystal, which is cooled, and the hot testpiece so that the transducer is shielded from the heat. This causesconsiderable difficulties in assuring the required acoustic contactbetween the test piece and the transducer, and the effective passage ofthe ultrasonic beam through the refractory heat shield. Fused salts havebeen used in an effort to overcome these difficulties but such use hascaused other and sometimes even greater ditficulties. None of the knownsystems of this type has proved altogether satisfactory.

The disadvantages of these conventional systems are further increasedwhen the surface area of the test piece to be inspected is rough orirregular. These disadvantages include difliculty of operation and ofhandling the system and, most of all, considerable irregularities in theacoustic contact due to the presence of oxide surface coatings onmetallic test pieces and a considerable absorption of ultrasonicvibrations in the couplant.

It is the primary object of the present invention to overcome thesedisadvantages and to permit ready ultrasonic inspection of test pieceswhose temperature is considerably above that to which theelectro-mechanical transducer of the system may be safely subjected.

With this and other objects in view, this invention provides anultrasonic inspection method wherein an electro-mechanical transducerface is placed adjacent an area of the test piece to be inspected and astream of a cold liquid couplant is continuously delivered between thetransducer face and this test piece area to couple the transduceracoustically to the area. The liquid of the couplant has a boiling pointbelow the temperature of the test piece and at least a portion of thestream of fresh liquid couplant entering the space between thetransducer face and the test piece surface first sweeps the transducerface before more closely approaching the test piece to prevent thetransducer face from being unduly heated by the test piece. The liquidcouplant stream forms a flowing liquid film in the space and moves atall points at such high speed that the liquid does not reach its boilingpoint in the space. A high-speed stream of a cooling liquid, which maybe the liquid couplant stream, reduces the surface temperature of thearea rapidly and fieetingly.

In one embodiment of the invention, the high-speed stream of the coolingliquid is delivered to a region of the test piece having an area atleast equal to the area of the test piece to be inspected, and thetransducer face is immediately thereafter placed adjacent the cooledregion while its surface temperature is still reduced, with a separatestream of a cold liquid couplant being delivered in the above-describedmanner. In this embodiment, the transducer face may be continuouslymoved over successive cooled regions of the test piece.

In another embodiment, the cooling liquid and liquid 3,303,691 Patented"Feb. 14, 1 967 couplant stream are the same, the high-speed liquidstreamserving the function of cooling liquid and liquid couplant.

Either the liquid couplant or the cooling liquid, or both, arepreferably water but if the temperature of the test piece is so highthat water cannot be utilized, liquids with a higher boiling point areused. Merely by way of example, such liquids include oils of differentboiling points or fused salts or metals. When such liquids are used, thetransducer must be enclosed in a cooled, for instance water-cooled,container, the cooling liquid simultaneously establishing the necessaryacoustic contact between the transducer and the container, while thecontainer is in acosutic contact with the test piece through the liquidcouplant with the high boiling point. It will be realized, however, thatthe number of separating interfaces through which the ultrasonic beammust pass in such a system is increased, which unfavorably influencesthe accuracy of the flaw detecting system. Therefore, water isdefinitely the preferred fluid medium.

The above and other objects and features of the present invention willbecome more apparent in the following detailed description of twoembodiments of the method, taken in conjunction with the accompanyingdrawing showing useful apparatus for carrying out the method and whereinFIG. 1 is a vertical section along line I-I of FIG. 2 of a searchingunit for carrying out the method according to one embodiment;

FIG. 2 is a bottom view of the unit of FIG. 1;

FIG. 3 is a sectional view along line IIIIII of FIG. 4 similar to thatof FIG. 1 of another embodiment; and

FIG. 4 is a bottom view of FIG. 3.

Referring now to the drawing and first to FIGS. 1 and 2, there is showna searching unit useful for the continuous inspection of hot flat stock,for instance rolled metal sheets.

In the illustrated embodiment, the metallic body 1 of the searching unithas a flat end face 10 designed to be placed parallel to, and at aslight distance of, for instance, a few tenths of a millimeter from, thefiat surface of the test piece 2 to be inspected. In the illustratedembodiment, the searching unit is hydraulically supported on the testpiece surface in a manner more particularly described and claimed in ourcopending application Serial No. 259,660, now Patent No. 3,159,756,filed on even date herewith and also entitled Ultrasonic Inspection.This support forms no part of the present invention and will, therefore,be described only sketchily herein. As shown, the end face 10 definesthree circular orifices at the end of hydraulic liquid delivery conduits3, 4, and 5 constituted by bores in the searching unit body 1 andpermitting three supporting colums of a hydraulic liquid to flowtherethrough to support the body end face 10 in spaced relation to thetest piece surface.

A reservoir 6 holding a liquid under a suitable pressure is bolted tothe top of the searching unit body 1 and receives the liquid through thesupply conduit 19, tho liquid being pumped or otherwise delivered to thereservoir under the desired ressure from a source (not shown). Withwater used as hydraulic liquid, we have found a head of 4 kg./sq. cm. tobe satisfactory but this may obviously vary according to operatingconditions and dimensions of the searching unit and delivery conduits.

As more fully explained in our above-mentioned copending application,whose disclosure is herewith made a part of the description, thehydraulic fluid is fed into the delivery conduits through suitablyrestricted passages of calibrated nozzles 7, 8 and 9, respectively, insuch a manner that the liquid fiow is controlled substantiallyindependently of the conditions of support of the searching unit on thetest piece and maintained substantially constant regardless of suchconditions. In such a hydraulic support, the searching unit floats onthe test piece and the distance on the end face 10 from the test piecesurface is self-regulated in sole dependence on the head of thehydraulic liquid in the supporting columns.

Ultrasonic inspection iseffected by an annular piezoelectric crystal 11pierced by a central hole 12 in alignment with conduit 3 to permitpassage of the hydraulic liquid to the surface of the test piece. Thepiezo-electric transducer is connected by a coaxial cable in the mannerillustrated in FIG. 3 to an ultrasonic generator (not shown) toconstitute an otherwise conventional ultrasonic flaw detection system asdescribed, for instance, in Ultrasonic Flaw Detection, issued by the U8.Department of Commerce, November 1958, and available through theSuperintendent of Documents, U.S. Government Printing Oflice. Since theinvention is not concerned with this aspect of the system, theelectrical circuit elements connected to the transducer have not beenillustrated.

In the illustrated embodiment, the hydraulic liquid fed through conduit3 simultaneously serves as a high-speed stream of coolant and as aliquid couplant between the transducer face and the area of the testpiece to be inspected. The stream of fresh liquid coming from reservoir6 radially flows outwardly from conduit 3 and sweeps the transducer faceto prevent the face from being heated. The stream fills the spacebetween the transducer face and the test piece area to be inspected toform therein a liquid couplant, and the liquid film moves at such highspeed that the liquid does not reach its boiling point in this space. Asclearly shown, the liquid flow in the space is unencumbered and thereare no dead spaces or eddies to interfere with the rapid andcontinuously even outward flow of the liquid over the entire transducerface and test piece area to be inspected.

A fourth conduit 13 connects the reservoir 6 with a V-shaped orifice 14in end face 10 of the searching unit body 1. The orifice 14 is locatedbetween the crystal 11 and the line which connects the orifices of theconduits 4, 5, and the apex of the V-shape is directed toward theafore-mentioned line. The conduit 13 delivers a wave or sheet of coolingliquid to the test piece surface, the flow of this cooling liquid beingcontrolled by another nozzle 15 which is threadedly mounted for readyreplacement in conduit 13, as nozzles 7, 8 and 9 are mounted in conduits3, 4 and 5, respectively. Thus, suitable calibration of the nozzlepassages controls the liquid flow through all conduits.

This searching unit operates in the following manner:

The searching unit and the test piece are moved relatively to each otherin the direction of arrow A, either by maintaining the test piece atrest and moving the searching unit over it in the indicated direction orby maintaining the searching unit at rest and moving the test piece pastit in the opposite direction. In either case, the cooling liquid wavedelivered through V-shaped orifice 14 rapidly and fleetingly reduces thesurface temperature of the [hot test piece before the transducer movespast the cooled region.

The hydraulic liquid delivered through conduits 3, 4 and flows as a filmunder the end face of the searching unit, the even distribution of theliquid being facilitated by the circular enlargements 16, 17 and 18forming the outlet orifices of the delivery conduits. The cold hydraulicliquid flowing out of delivery conduits 4 and 5 causes a first intensivecooling of the test piece surface. This cooling is completed by thecooling liquid flowing out of V-shaped orifice 14. The inclination ofthe two branches of the V-shaped orifice causes the cold wave of liquidemerging therefrom like a jet to sweep before it the liquid filmproduced by the hydraulic liquid coming from conduits 4 and 5, and whichhas been heated by contact with the hot test piece surface, as therelative movement in the direction of arrow A proceeds. This V-shapedcold liquid jet thus sweeps the hot liquid laterally outwardly andout ofthe path of the immediately following transducer which will inspect thethusly cooled region of the test piece while the surface temperature isstill reduced.

Simutaneously, the liquid flowing through conduit 3 is distributedradially and flows under centrifugal force and in a homogeneous streamunder the transducer face with a speed sufiicient to prevent the cooledsurface from being reheated by the internal heat 'of the test piece andthe rapidly flowing liquid from reaching the boiling point in the pathof the ultrasonic beam. At the same time, this rapidly flowing liquidcouplant cools the face of the transducer.

The embodiment of FIGS. 3 and 4 is a simple searching unit useful forcarrying out the method of this invention when a fixed point of a testpiece is to be inspected. The piezo-electric transducer 11a is similarto that of the embodiment of FIG. 1 and is suitably arranged in amounting supported on a test piece (not shown) by three points ofsupport 22, 23, 24 forming a tripod support for the transducer on thetest piece, the face of the annular transducer being adjacent an area ofthe test piece to be inspected. The transducer defines a central hole12a in alignment with an axial conduit 3a in the transducer mounting. Asconventional, the transducer is connected with conventional elements ofan electric generator and indicating circuit (not shown) by a coaxialcable 21 coupled to the mounting at 20. In op eration, the transducermounting is placed on its tripod support over the area of the test pieceto be tested and a suitable cold liquid, such as water, is suppliedunder pressure to the delivery conduit 3a through connection 19a leadingto a supply of water under the required head, similarly to theembodiment of FIG. 1. If the test area is hot, the jet of cold liquidflowing through the hole 12a cools the surface and is radiallydistributed in an outward fiowof considerable speed between the face ofthe transducer and the test piece area adjacent thereto. liquid pressureis high enough to assure a sufliciently rapid outward flow, the liquidfilm constituted by the rap idly flowing stream of liquid willsimultaneously cool the transducer face and the test piece area adjacentthereto whileacoustically coupling the transducer to this area. Thespeed of the liquid flow is so chosen that the liquid will not boil inthe space traversed by the ultrasonic beam.

With a piezo-electric transducer 11a, 30 millimeters When thetemperatureof the test piece was 800 C. a rate of flow of water of 3.2 gallons perminute was necessary. In that case the transducer was puton the testpiece and it wasnecessary to wait about two seconds be-' foretheapparatus started to operate.

It is easy to understand that it is always'necessary to have a high flowspeed in order that the layer of steam Of course the speed of the becarried away by 'water. cooling fluid that is necessary depends on manyfactors; shape and dimensions of the transducer, distance between thelatter and the test piece, said distance varying as the wave length,nature of the surface, etc.

The considerable advantages in the ultrasonic inspection of hot testpieces obtained by the method of the pres'-' ent invention will beobvious from the above description of certain specific embodimentsthereof and they will be most pronounced when the test piece surface isrough, irregular or covered by a coating of oxide, as for instance inmetallurgically produced objects. In the conventional systems, wherein arefactory material is interposed between the transducer and the testpiece, the temperature gradient between the transducer and the testpiece is If the 1 formed in the refractory material and the temperatureof the test piece is practically unchanged.

According to the invention, the area of the test piece to be inspectedis rapidly and fleetingly cooled down by a localized cooling jet, thesurface of this area assuming a low enough temperature for the time thatthe transducer face is opposite it to prevent the liquid couplant fromboiling although its boiling point is lower than the general temperatureof the test piece. This is accomplished by interposing between thetransducer and the test piece surface a liquid film moving at greatspeed so that the thermal inertia of the liquid produces a non-boilingzone in front of the transducer face. In this manner, ultrasonic testingis possible because the liquid couplant can fulfill its function ofacoustically coupling the test piece with the electro-mechanicaltransducer while the transducer remains protected from the hightemperature of the test piece.

Due to the rapid flow of the liquid interface, the only heating of thetransducer face can be effected by convec tion. To accomplish this, itis most important to avoid any dead or eddy zones in the space betweenthe transducer and the test piece so that the flow speed of the liquidis nowhere reduced but proceeds at all points so rapidly as to preventthe liquid from boiling. A relatively homogeneous stream of liquid inall directions to form a rapidly moving liquid film meets thiscondition.

At the same time, this fleeting surface cooling of a limited area of thetest piece does not substantially modify the physical characteristics ofthe test piece, which rae due to its elevated temperature. Theconsiderable internal heat of the test piece suflices to reestablish thesurface temperature rapidly to at least nearly its original degree atthe tested area as soon as the inspection is completed and the searchingunit is moved on and away from this area. Therefore, the ability of thetested piece to be subjected to such finishing operations as rolling,clipping, shaving, stamping or any other operation requiring a certainplasticity of the hot meta-l, is in no way impaired.

Water will obviously be the most useful and practical cooling andcouplant liquid, and it will be used wherever feasible. Preliminaryexperiments have shown that water can be successfully used in the methodof the present invention for test piece temperature up to about 500 C.or 600 C. and it is quite likely that further experiments may establishits usefulness even beyond these operating temperatures. Dependingprimarily on the control of the water pressure and the correspondingspeed of flow of the water, a non-boiling zone may still be maintainedin the path of the ultrasonic beam. Outside of this Zone, the water willboil and evaporate but the vapor production is favorably influenced bythe heating phenomenon. In effect, the vapor volume is not considerablylarger at 1000 C. than it is at 300 C. The only essential condition isthe avoidance of vaporization of the couplant in front of the transducerbecause this would gravely disturb the acoustic contact and testingwould become impossible if the ultrasonic beam would have to passthrough vapor and bubbles.

Another advantage of the invention in the inspection of iron metal testpieces resides in the fact that the sudden contact of a cold liquid jetahead of the transducer with the hot test piece surface tends to cause athermal shock on the oxide layer generally present on such pieces athigh temperatures. This thermal shock will tend to crack and detach theoxide pellicle and to sweep it away by the dynamic force of the liquidbefore the transducer reaches the test area.

While the invention has been particularly described in connection with apreferred embodiment, it will be clearly understood that many variationsand modifications may occur to the skilled in the art without departingfrom the spirit and scope of this invention as defined in the appendedclaims.

What is claimed is:

1. In a method of ultrasonically inspecting a test piece having anelevated temperature, the steps of placing an electro-mechanicaltransducer adjacent a test piece to be inspected so that a face of thetransducer is oppositely spaced from a surface area of the test piececontinuously delivering a stream of a cold liquid couplant between thetransducer face and said area to couple the transducer acoustically tosaid area, the liquid of the couplant having a boiling point below thetemperature of the test piece, at least a portion of the stream of freshliquid couplant first sweeping said face of the transducer before moreclosely approaching said area of the test piece to prevent thetransducer face from being heated by the test piece area, said liquidcouplant stream forming a flowing liquid film in said space moving atall points of said space at such high speed-that the liquid does notreach its boiling point in said space.

2. In the method of claim 1, said stream being water.

3. In the method of claim 1, said portion of said stream being water.

4. In the method of claim 1, a portion of said stream being water.

5. In a method of ultrasonically inspecting a test piece having anelevated temperature: the steps of delivering a high-speed stream of acooling liquid to a surface region of the test piece to reduce thetemperature of said region, immediately thereafter, and while saidsurface temperature is reduced, placing a face of an electromechanicaltransducer oppositely and spacedly adjacent said region, continuouslydelivering a stream of a cold liquid couplant between the transducerface and said region to couple the transducer acoustically to said testpiece, the liquid of the couplant having a boiling point below saidelevated temperature, at least a portion of the stream of fresh liquidcouplant first sweeping said face of the transducer to prevent thetransducer face from being heated before the stream more closelyapproaches said region of the test piece, the liquid couplant streamforming a flowing liquid film in said space and moving at all points ofsaid space at such high speed that the liquid does not reach its boilingpoint in said space.

6. In the method of claim 5, the liquid couplant and the cooling liquidbeing water.

7. In a method of ultransonically inspecting a test piece having anelevated temperature: the steps of placing an electro-mechanicaltransducer face oppositely adjacent a surface area of the test piece tobe inspected, continuously delivering a high-speed stream of a coldliquid couplant between the transducer face and said area of the testpiece whereby the transducer is acoustically coupled to said area, theliquid having a boiling point below said elevated temperature, at leasta portion of the stream of the fresh liquid first sweeping thetransducer face to prevent the transducer face from being heated beforethe liquid more closely approaches said test piece area, the liquidstream forming a flowing liquid couplant in said space and moving at allpoints of said space at such high speed that the liquid does not reachits boiling point in said space.

8. In the method of claim 7, the liquid being water.

9. A method of ultrasonically inspecting a test piece having an elevatedtemperature which comprises:

(a) placing an eletromechanical transducer adjacent a surface portion ofthe test piece to be inspected in such a manner that a face of thetransducer and said surface portion define a space therebetween;

(b) passing a stream of a liquid couplant through said space so as tofill the same,

(1) the boiling point of said liquid couplant being lower than saidelevated temperature, and

(2) the rate of movement of said stream in said space being sufficientto hold the temperature of each portion of said couplant in said spacebelow said boiling point; and

(c) passing an ultrasonic beam between said transducer and said testpiece throughthe liquid in said space.

10. An apparatus for ultrasonicallyinspecting a test piece havinganelevated temperature comprising, in combination:

(a) a supporting body having a face;

(b) an electromechanical transducer mounted on said body and exposed onsaid face,

(1) said body being formed with a plurality of openings in said face;and

(0) means for discharging a liquid from each of said openings,

(1) two of said openings defining a line in said face spaced from saidtransducer,

(2) a third opening in said face being V-shaped and interposed betweensaid transducer and said line, the apex of the V-shape being directedtoward said line.

References Cited by the Examiner UNITED STATES PATENTS 2,873,391 2/1959Schulze 7367.9 X 2,992,553 7/1961 Joy 7367.8 3,121,325 2/1964 Rankin etal 7367.8 X 3,171,047 2/1965 Bergman et a1 3108.7 3,242,723 3/1966 Evans7371.5

FOREIGN PATENTS 1,282,845 12/1961 France.

RICHARD C. QUEISSER, Primary Examiner.

J. B. BEAUCHAMP, Assistant Examiner.

9. A METHOD OF ULTRASONICALLY INSPECTING A TEST PIECE HAVING AN ELEVATEDTEMPERATURE WHICH COMPRISES: (A) PLACING AN ELETROMECHANICAL TRANSDUCERADJACENT A SURFACE PORTION OF THE TEST PIECE TO BE INSPECTED IN SUCH AMANNER THAT A FACE OF THE TRANSDUCER AND SAID SURFACE PORTION DEFINE ASPACE THEREBETWEEN; (B) PASSING A STREAM OF A LIQUID COUPLANT THROUGHSAID SPACE SO AS TO FILL THE SAME, (1) THE BOILING POINT OF SAID LIQUIDCOUPLANT BEING LOWER THAN SAID ELEVATED TEMPERATURE, AND (2) THE RATE OFMOVEMENT OF SAID STREAM IN SAID SPACE BEING SUFFICIENT TO HOLD THETEMPERATURE OF EACH PORTION OF SAID COUPLANT IN SAID SPACE BELOW SAIDBOILING POINT; AND (C) PASSING AN ULTRASONIC BEAM BETWEEN SAIDTRANSDUCER AND SAID TEST PIECE THROUGH THE LIQUID IN SAID SPACE.